Because of the large difference in temperature between the superconductor element and the equipment for connecting to said element, i.e. between ambient temperature and cryogenic temperature that may be about −200° C., it is necessary to interpose a connection structure between the element and the equipment in order to make the temperature transition while minimizing heat losses, and while nevertheless complying with electrical constraints due for example to the high voltage when a cable is involved. This structure comprises an electrical bushing made up of mainly of a central conductor surrounded by an insulating sheath, for transporting electricity from the superconductor cable to an outlet connection at ambient temperature. This structure must achieve the temperature transition over a reasonable length while ensuring that heat conduction losses along the electrical bushing are low so as to avoid boiling the cryogenic liquid that cools the cable. The section of the central conductor must therefore not be too great. Nevertheless, a high electric current can lead to heat losses due to the conductor becoming heated by the Joule effect, and under such circumstances it is advantageous to increase the section of the central conductor. There are thus two conflicting requirements.
Another technical problem to be solved is that of controlling the distribution of the electric field created by the medium or high voltage of the central conductor of the electrical bushing so as to avoid breakdowns.
The known solution to the problem of heat losses consists in providing the connection structure with an adiabatic intermediate enclosure, an airlock or “buffer” enclosure so to speak, placed between the portion at cryogenic temperature and the portion of the connection structure at ambient temperature. The electrical bushing passes through the intermediate enclosure. That solution is described in European patent application EP 1 283 576, for example. The side walls of the intermediate enclosure are constituted by the side walls of a cryostat. The bottom and top walls include fastening flanges through which the electrical bushing passes, the bottom wall being adjacent to the cryogenic temperature portion and the top wall being adjacent to the ambient temperature portion. The intermediate enclosure is either evacuated, or else filled with a gas that provides good insulation both thermally and electrically. The vacuum level or the gas composition needs to be selected in order to achieve both kinds of insulation. The outside wall of the intermediate enclosure is connected to ground potential.
The drawback of that solution lies in the need for good sealing of the intermediate enclosure, and in particular of the places where the electrical bushing passes through the bottom and top walls, thus leading to manufacturing constraints that are difficult and expensive. For example, even a very small leak between the cryogenic temperature portion and the intermediate enclosure (e.g. a leak of about 10−8 millibars per liter second (mbar/L.s)) inevitably leads to a change in the composition of the gas or to a deterioration in the vacuum level in the intermediate enclosure. If the cryogenic fluid is liquid nitrogen, a leak leads to the presence of gaseous nitrogen in the intermediate enclosure, thus leading firstly to additional consumption of liquid nitrogen, and secondly to a decrease in the thermal and electrical insulation of the intermediate enclosure. Excess pressure in the intermediate enclosure as a result of such a leak is also unsuitable for being controlled by means of safety valves since opening a safety valve would lead to the destruction of the thermal and dielectric insulating medium (vacuum or gas).